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July 29, 2016

Why Deleted Files Can Be Recovered

When you a delete a file, it isn’t really erased – it continues existing on your hard drive, even after you empty it from the Recycle Bin. This allows you (and other people) to recover files you’ve deleted.

If you’re not careful, this will also allow other people to recover your confidential files, even if you think you’ve deleted them. This is a particularly important concern when you’re disposing of a computer or hard drive.

Windows (and other operating systems) keep track of where files are on a hard drive through “pointers.” Each file and folder on your hard disk has a pointer that tells Windows where the file’s data begins and ends.

When you delete a file, Windows removes the pointer and marks the sectors containing the file’s data as available. From the file system’s point of view, the file is no longer present on your hard drive and the sectors containing its data are considered free space.

However, until Windows actually writes new data over the sectors containing the contents of the file, the file is still recoverable. A file recovery program can scan a hard drive for these deleted files and restore them. If the file has been partially overwritten, the file recovery program can only recover part of the data.

Note that this doesn’t apply to solid-state drives (SSDs) – see below for why.

If you’re wondering why your computer doesn’t just erase files when you delete them, it’s actually pretty simple. Deleting a file’s pointer and marking its space as available is an extremely fast operation. In contrast, actually erasing a file by overwriting its data takes significantly longer. For example, if you’re deleting a 10 GB file, that would be near-instantaneous. To actually erase the file’s contents, it may take several minutes – just as long as if you were writing 10 gigabytes of data to your hard drive.

To increase performance and save time, Windows and other operating systems don’t erase a file’s contents when it’s deleted. If you want to erase a file’s contents when it’s deleted, you can use a “file-shredding” tool – see the last section for more information.

Solid-State Drives Work Differently: None of this applies to solid state drives (SSDs). When you use a TRIM-enabled SSD (all modern SSDs support TRIM), deleted files are removed immediately and can’t be recovered. Essentially, data can’t be overwritten onto flash cells – to write new data, the contents of the flash memory must first be erased. Your operating system erases files immediately to speed up write performance in the future – if it didn’t erase the file data immediately, the flash memory would first have to be erased before being written to in the future. This would make writing to an SSD slower over time.
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